In pre-amplifier circuits, there is often a need to amplify and convert a periodic component of an input current signal to a voltage equivalent. Such pre-amplifiers may also be required to block any D.C. component on which the periodic input signal may be superimposed and simultaneously clamp the voltage equivalent output signal to ground.
While these functions may be implemented in a variety of ways, in general such means commonly utilize a DC blocking capacitor followed by a linear amplifier and a diode clamp. Since the blocking capacitor must also pass the input periodic signal being amplified, in the case of low frequency input signals, the capacitance required may assume prohibitively large values. Such low frequency input signals are commonly encountered in photometric applications for bichromatic analysis in which a light beam is mechanically interrupted or chopped at a relatively low periodic rate and thereafter converted by a photomultiplier tube to a corresponding signal for processing and analysis.
Pre-amplifiers currently in use for such photometric applications have eliminated the need for a DC blocking capacitor. Present feedback amplifiers sample the photomultiplier dark current signal level, i.e., the quiescent direct current level of the photomultiplier tube (PMT) signal output.
This DC level is used in the feedback network to charge a capacitor through a diode and resistor combination in a direction to remove the offset voltage due to the dark current signal present at the output of the PMT pre-amplifier. The feedback amplifier is normally in a state of positive saturation, with brief, uncontrolled excursions through its linear operating region, at times ending in negative saturation, with equally uncontrolled return excursions to positive saturation. The highly non-linear, frequency independent response of the feedback network results in an undesirable overall response to input noise. Because of the overall circuit configuration, increasing the network's response time inevitably leads to other more disastrous trade-offs.
The feedback network is thus incapable of frequency discrimination and output noise is processed exactly like the signal. As a result, the most negative excursion of signal plus noise at the output of the pre-amplifier is clamped to ground. This induces a noise dependent offset voltage in the pre-amplifier output signal. It is a characteristic of photomultiplier tubes that their noise level changes as a function of increasing tube operating voltage. Since the voltage applied to the photomultiplier tube is a function of the absorbance value to be measured during bichromatic analysis, the noise dependent offset voltage at the output of the pre-amp inevitably leads to non-linearity measurement errors whose magnitude is related to the noise characteristics of the photomultiplier tube Thus, in the past, in order to minimize the error due to the noise dependent offset voltages, it was required to pre-test and therefore pre-select the photomultiplier tubes having a low range of noise level changes. Under normal circumstances, the rejection rate of new photomultiplier tubes in order to meet this criteria amounted to 50-60%.
It is therefore desirable to provide a pre-amplifier able to restore ground in the presence of noise, and particularly one capable of compensating for the dark current output of a photomultiplier tube without requiring time consuming and costly pre-testing of photomultipliers to determine those having a desired low range of noise level changes.